Principal results

Site U1311 is located on a small knoll near the break in slope where the corrugated dome meets the adjacent volcanic hanging wall block (see Fig. F2 in the “Site U1309” chapter and Fig. F2 in the “Expedition 304/305 summary” chapter). The hole was cited based on the possibility that the knoll represents a thin klippe of basalt above the detachment surface. Recovery of fresh glassy vesicular basalt in Hole U1311A does not provide a definitive test of this hypothesis. The eruptive basalt could have come from the inferred hanging wall, as hypothesized, or it could have been erupted onto or against the slope of the easternmost surface of the dome at any time after exposure at the seafloor.

Hole U1311A (30°10.61′N, 42°04.19′W; 2552 meters below sea level) is located on the southern slope of the knoll. A 60 m × 60 m survey with the vibration-isolated television camera documented a ~3600 m2 area of mud- and rubble-covered seafloor, with a moderate slope to the south-southeast. Along the northeastern corner of the survey area, a moderately to steeply southeast dipping, >20 m high scarp oriented east-northeast (~75°) crops out and is characterized by rounded pillow structures.

One attempt to drill at this site penetrated 12 meters below seafloor, including 3.5 m of unconsolidated mud. Recovery from the hole produced 1.5 m (13%) of fresh, vesicular, moderately plagioclase-olivine phyric basalt pillows, with sparse glass preserved. Almost all pieces are angular and have broken along fracture surfaces within pillows—few, if any, were cut by the drill. The fracture surfaces are discolored dark brown and minimally altered. These observations suggest that the core is derived from in situ pillows. Some piece interiors include multiple gray Liesegang bands, indicating pervasive minor or incipient alteration. Vesicles close to fracture surfaces are internally discolored brown but not filled.

The basalt is dark gray to black in color and characterized by abundant (5%–10%) seriate plagioclase that locally occurs in radiating clumps. Sparse euhedral plagioclase phenocrysts range upward in size to ~2 mm. In thin sections taken from pillow interiors, the basalt appears fresh, with minimal darkening of the matrix and local orange to green smectites either in vesicles or in the vicinity of olivine (see Fig. F3 in the “Expedition 304/305 summary” chapter). Randomly oriented acicular to prismatic plagioclase, ranging in size from <0.1 to 0.5 mm, makes up ~40% of the sample. Many of the plagioclase crystals have a hollow or swallowtail quench morphology. Olivine microphenocrysts (~5%) appear either in subophitic crystal clots with plagioclase or as euhedral microphenocrysts ranging in size from 0.1 to 0.3 mm and in shape from larger prisms to smaller diamonds and more complex quenched forms. The remainder of the rock is the devitrified glass matrix, dominated by plumose quenched clinopyroxene with anhedral interstitial plagioclase. Oxide intervals are abundant, up to 2%, in the matrix and are mostly present as complex quench morphologies, most likely ilmenite. Vesicles occupy 3%–5% of total volume. They are present in two forms, round (~0.1 mm) and elongate, and are irregular and locally interconnected. Most are unfilled, but a few are completely or partially filled by devitrified glass, and a few, especially near fracture surfaces, are filled by secondary green or orange smectite. They generally range in size up to 0.5 mm but in places are as large as 2 mm. Where present, glass is 1–3 mm thick with 50%–100% palagonitization close to outer surfaces.